Alessandro Erta

Performance Evaluation of the IEEE 802.16 MAC for QoS Support

The IEEE 802.16 is a standard for broadband wireless communication in metropolitan area networks (MAN). To meet the QoS requirements of multimedia applications, the IEEE 802.16 standard provides four different scheduling services: unsolicited grant service (UGS), real-time polling service (rtPS), non-real-time polling service (nrtPS), and Best Effort (BE). The paper is aimed at verifying, via simulation, the effectiveness of rtPS, nrtPS, and BE (but UGS) in managing traffic generated by data and multimedia sources. Performance is assessed for an IEEE 802.16 wireless system working in point-to-multipoint (PMP) mode, with frequency division duplex (FDD), and with full-duplex subscriber stations (SSs). Our results show that the performance of the system, in terms of throughput and delay, depends on several factors. These include the frame duration, the mechanisms for requesting uplink bandwidth, and the offered load partitioning, i.e., the way traffic is distributed among SSs, connections within each SS, and traffic sources within each connection. The results also highlight that the rtPS scheduling service is a very robust scheduling service for meeting the delay requirements of multimedia applications

A downlink data region allocation algorithm for IEEE 802.16e OFDMA

IEEE 802.16e specifies a connection-oriented centralized medium access control (MAC) protocol, based on time division multiple access (TDMA), which adds mobility support to the MAC protocol defined by the IEEE 802.16 standard for fixed broadband wireless access. To this end, orthogonal frequency division multiple access (OFDMA) is specified as the air interface. In OFDMA, the MAC frame extends over two dimensions: time, in units of OFDMA symbols, and frequency, in units of logical sub-channels. The base station (BS) is responsible for allocating data into the MAC frames so as to meet the quality of service (QoS) guarantees of the admitted connections of the mobile stations (MSs). This is done on a frame-by-frame basis by defining the content of map messages, which advertise the position and shape of data regions reserved for transmission to/from MSs. We refer to the latter operation as data region allocation. In this paper, we propose a sample data region allocation algorithm (SDRA), and we evaluate its performance by means of Monte Carlo analysis. The effectiveness of SDRA is assessed in several scenarios, involving mixed voice over IP (VoIP) and best effort MSs, different modulations, and frequency re-use plans.

A General Interference-Aware Framework for Joint Routing and Link Scheduling in Wireless Mesh Networks

Joint design and optimization of traditionally independent problems such as routing and link scheduling have recently become one of the leading research trends in wireless mesh networks. Although technically challenging, cross-layering is, in fact, expected to bring significant benefits from the network resource exploitation standpoint to achieve high system utilization. In this article we propose a versatile framework for joint design of routing and link scheduling, introducing the notion of link activation constraints, which are related to the transceiver capability and the broadcast nature of the wireless medium. To this end, we introduce a taxonomy of wireless interference models to harmonize existing approaches presented in the literature. Finally, we evaluate the impact on network capacity of the various interference models when optimal joint routing and link scheduling are employed.

Performance evaluation of the mesh election procedure of ieee 802.16/wimax

IEEE 802.16 is a recent standard for Broadband Wireless Access networks, which includes a mesh mode operation for distributed channel access of peering nodes. In accordance with the IEEE 802.16 MAC protocol, time is partitioned into frames of fixed duration, each one divided into two sub-frames, for control and data transmission, respectively. Slots in the control sub-frame are used by nodes to negotiate the schedule of transmissions in data sub-frames, and are accessed by means of a collision-free distributed procedure, namely the mesh election procedure. In this paper, we analyze the performance of the mesh election procedure by means of extensive simulations, and identify the system configuration parameters that have the most impact on the performance of control message transmission. The analysis is carried out under the assumption that the wireless link is error-free.

Bandwidth Allocation with Half-Duplex Stations in IEEE 802.16 Wireless Networks

IEEE 802.16 is a recent IEEE standard for broadband wireless access networks. In IEEE 802.16 networks, the medium access control (MAC) protocol is centralized and explicitly supports quality of service (QoS). That is to say, access to the medium by a number of subscriber stations (SSs) is centrally controlled by one base station (BS), which is responsible for allocating bandwidth to several MAC connections in order to provide them with the negotiated QoS guarantees. However, although the network can be operated in frequency division duplex (FDD) mode (that is, transmissions from the BS (downlink) and SSs (uplink) occur on separate frequency channels), the standard supports SSs with half-duplex capabilities. This means that they are equipped with a single radio transceiver which can be used either to transmit in the uplink direction or to receive in the downlink direction. This may severely hamper the capacity to support QoS. Therefore, in order to allocate bandwidth, an IEEE 802.16 BS has to solve two related issues: (1) how it can schedule bandwidth grants to SSs in order to meet the QoS requirements of their connections and (2) how it can coordinate the uplink and downlink scheduled grants so as to support half-duplex capabilities. In this paper, we derive sufficient conditions for a set of scheduled grants to be allocated so that the transmission of each half-duplex SS does not overlap with its reception. Based on this, we propose a grant allocation algorithm, namely, the half-duplex allocation (HDA) algorithm, which always produces a feasible grant allocation provided that the sufficient conditions are met. HDA has a computation complexity of 0(n), where n is the number of grants to be allocated. Finally, we show that the definition of HDA allows us to address the two issues mentioned above by following a pipeline approach. This is when scheduling and allocation are implemented by separate and independently running algorithms, which are just loosely coupled with each other. We show via extensive simulations that the performance of SSs with half-duplex capabilities, in terms of the delay of real-time and non-real-time interactive traffic, using HDA almost perfectly matches that of full-duplex SSs, whereas an alternative approach, based on the static partitioning of half-duplex SSs into separate groups, which are allocated alternately, is shown to degrade the performance.

End-to-End Bandwidth Reservation in IEEE 802.16 Mesh Networks

IEEE 802.16 mesh does not include support for traffic flows with strict quality of service requirements. In this paper, we propose an end-to-end bandwidth reservation protocol (EBRP) in the backhaul of a wireless mesh network using IEEE 802.16 mesh. The distinctive feature of EBRP is that it is carried out at the medium access control (MAC) layer. Therefore, EBRP not only makes the resource reservation process extremely rapid, but it also allows the resources available to be allocated efficiently by exploiting technology-specific information available at the MAC. We present EBRP as part of a framework which also includes support for distributed call admission control (CAC). Preliminary simulation results obtained with VoIP traffic and nodes arranged in a grid topology are presented to show the effectiveness of EBRP under controlled channel conditions.

A physical model scheduler for multi-hop wireless networks based on local information

There is wide consensus that properly taking into account wireless interference is necessary to design high performance link scheduling algorithms for multi-hop networks. However, most approaches in the literature use simplified models, which significantly abstract from the physical behavior of wireless links. Indeed, the main problem in representing the wireless propagation conditions with a proper level of detail is the very large amount of information needed, that includes the wireless link gains between all node pairs. In this paper, we propose a greedy, centralized scheduler which is based on the physical interference model but aims at exploiting local information available at each node, in order to reduce global information exchange and therefore the overhead as well as the computational complexity of the algorithm. We prove the effectiveness of our approach by extensive simulation results. We also show that our system outperforms the most up-to-date benchmark in realistic interference aware schedulers for wireless multi-hop networks.

Improved network resilience of wireless mesh networks using MPLS and Fast Re-Routing techniques

The performance of a wireless mesh network (WMN) can be significantly affected by node failures and medium-term time-varying phenomena in the wireless channel, which may lead to significant interruptions in packet delivery service. Because of their nature, WMNs are often deployed with an inherent degree of redundancy. In principle, this flexibility might be exploited to cope with the link failure problem by temporarily re-routing ongoing traffic on alternate paths. However, the existing routing protocols cannot be used for this purpose, since they either rely on link-state updates, which happen on a longer time scale than the phenomena under consideration, or the decisions are taken by the source node, which does not know the status on intermediate links. In this paper we present a Fast Re-Routing enabled architecture that is optimized for WMNs and allows nodes to react to local node or link failures by activating a pre-configured alternate path to reach a two-hop neighbor. The proposed scheme is enabled by label switching/stacking. Alternate paths are created automatically during the path setup phase via a lightweight protocol derived from LDP. The solution is proved to be effective through a testbed implementation.

A Downlink MAC Frame Allocation Framework in IEEE 802.16e OFDMA: Design and Performance Evaluation

The IEEE 802.16e standard specifies a connection-oriented centralized Medium Access Control (MAC) protocol, based on Time Division Multiple Access (TDMA), which adds mobility support defined by the IEEE 802.16 standard for fixed broadband wireless access. To this end, Orthogonal Frequency Division Multiple Access (OFDMA) is specified as the air interface. In OFDMA, the MAC frame extends over two dimensions: time, in units of OFDMA symbols, and frequency, in units of logical sub-channels. The Base Station (BS) is responsible for allocating data into the frames so as to meet the Quality of Service (QoS) guarantees of the Mobile Stations’ (MSs) admitted connections. This is done on a frame-by-frame basis by defining the content of map messages, which advertise the position and shape of data regions reserved for transmission to/from MSs. We refer to the process of defining the content of map messages as frame allocation. Through a detailed analysis of the standard, we show that the latter is an overly complex task. We then propose a modular framework to solve the frame allocation problem, which decouples the constraints of data region allocation into the MAC frame, i.e. the definition of the position and shape of the data regions according to a set of scheduled grants, from the QoS requirements of connections. Allocation is carried out by means of the Sample Data Region Allocation algorithm (SDRA), which also supports Hybrid Automatic Repeat Request (H-ARQ), an optional feature of IEEE 802.16e. Finally, we evaluate the effectiveness of SDRA by means of Monte Carlo analysis in several scenarios, involving mixed Voice over IP (VoIP) and Best Effort (BE) MSs with varied modulations, with different sub-carrier permutations and frequency re-use plans.